Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
(OP)
Like many refiners, we currently vent the non condensibles from the vacuum tower eductor train to a fired heater. My understanding is that the EPA will be mandating in the future that we scrub this of H2S prior to burning it. Therefore, we need some motive pressure to accomplish this. I had initially thought to slip this stream into the FCC Wet Gas Compressor to take advantage of the existing amine scrubbing system. The problem that I see is that should we develop a leak of sufficient size on the vacuum system we would be sending enough air down the line to be in the explosive regime even though we would know it quickly due to loss of vacuum on the tower. I am leaning to an O2 monitor on the line, response time of the analyzer for line sizing, and a shutoff/diversion valve to address this problem. I am wondering what other solutions have been employed as I suspect others may have already addressed this problem.





RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
Best regards
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
You could use the O2 detector on the amine system to check that it is operating below the maximum safe oxygen content. Even then absorbing H2S in the scrubber will change the composition and you need to check if the gas would enter the flammable region.
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
I think gelpino has a good idea. Something like a venturi scrubber with recirculated amine as the scrubbing medium. Or you just continue with your original idea, but you are still going to have to re-compress the vacuum tower vent gas to get it over to the wet-gas compressor. I am assuming you have steam jets on the vacuum tower. If you have vacuum pumps then maybe you could get into the wet gas compressor. You probably would want some type of reliable interlock in the system should you ever lose tower vacuum though.
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
If the non-condensible is air, then EQUILIBRIUM hydrocarbons (HC) will be present. The vent composition is independent of air leakage rate. More air leaking into the system means that more HC is in vent gas. The composition stays constant. So an oxygen analyzer is unable to detect a change in leak rate.
You appear to be referring to a situation with the non-condensibles consisting of air and process gases. In this case composition varies with air leakage rate, because of dilution from the base load of process gas. It will be hard to infer changes in leak rates when the process gas load is more than 3 times air rate because there is so much dilution.
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
I looked into the liquid eductor a while back and don't think I can get the discharge pressure I would need.
I am going to use the O2 analyzer and send it to the FCC wet gas compressor. Basically, I know the max gas rate by the last eductor stage. So %O2 from an analyser tells me what the O2 potential flow could be and knowing the wet gas compressor absorber off gas typical flow and composition then I can determine a max O2 upstream before LEL at the highest concentration of non condensibles downstream and divert the flow so I don't ruin the compressor or something else downstream, if you know what I mean.
I don't know but the stack gas on my fired heaters is about 3 to 5% O2. 10% seems high and costly. Though you need to check design info from the heater vendor.
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
RE: Scrubbing Vacuum Tower Vent Gas/Safety Concerns of Air Intrusion
Try also to contact www.hijet.com...it's possible they have good news for you....but i'm not sure